1. Field of the Invention
The invention relates to an image forming apparatus adapted to form a toner image on an image bearing member, and transfer this toner image to a recording material, and particularly to an image forming apparatus of an electrophotographic type or an electrostatic recording type.
2. Description of Related Art
As a plural-color or full-color image forming apparatus of a conventional electrophotographic type, in recent years, there has been proposed an image forming apparatus of an intermediate transfer type in which toner images of respective colors formed on a photosensitive drum are successively superposed on an intermediate transfer member (intermediate transfer belt) to thereby form a color image, which is then collectively transferred to a recording sheet.
In this intermediate transfer type, a toner image is formed on the photosensitive drum by charging means, exposing means and developing means disposed around the photosensitive drum, and is electrostatically transferred to the intermediate transfer belt by transferring means in a primary transferring portion. When a color image is to be formed, toner images are successively transferred to the intermediate transfer belt, whereby a full-color image can be formed on the intermediate transfer belt.
The toner image transferred to the intermediate transfer belt is conveyed to a secondary transferring portion by the rotation of the intermediate transfer belt, and is electrostatically transferred to a recording medium. As methods of removing toner not transferred to the recording medium at this time, but remaining on the intermediate transfer belt, there have been proposed a method of pushing a cleaning blade against the intermediate transfer belt to thereby remove the remaining toner, a method of applying a bias to fur brush cleaning means to thereby electrostatically remove the remaining toner, etc.
The afore described fur brush cleaning is advantageous to such problem as the influence upon the life of the intermediate transfer belt which poses a problem in the blade cleaning means, or the load fluctuation by the fluctuation of frictional resistance, but the untransferred remaining toner after the secondary transfer includes some toner charged to the plus (+) polarity by the secondary transfer bias and some toner charged to the minus (−) polarity by the secondary transfer bias and therefore, there arises the problem that not all of the untransferred remaining toner can be completely collected by a single fur brush and by the application of a bias of one polarity. Against this problem, there has been proposed a method of applying bias voltages of different polarities, i.e., plus (+) and minus (−) polarities, to a plurality of fur brushes to thereby collect untransferred remaining toner after the secondary transfer (Japanese Patent Application Laid-open No. 2002-207403).
In such an image forming apparatus as described above, it sometimes happen that the image forming operation is interrupted by the faulty conveyance or the like of a recording material while the toner image on the photosensitive drum is being transferred onto the intermediate transfer belt, or while the toner on the intermediate transfer belt is being transferred to a recording sheet. In this case, the image forming apparatus has the return sequence of eliminating the cause, and thereafter performing the returning operation of collecting a toner image already developed on the photosensitive drum, or the toner image already transferred onto the intermediate transfer belt by cleaning means to thereby prepare for the next image formation.
In the toner image transferred onto the intermediate transfer belt at the point of time whereat the image forming has been interrupted, if it is of full-color, there exist toner images of maximum n colors (n>1) comprising monochromatic toner images on the photosensitive drum superposed one upon another. It is necessary to collect these toner images by the return sequence, but the electrostatic fur brush cleaning, unlike a cleaning blade, is limited in the amount of toner it can collect.
In the electrostatic fur brush cleaning, for example, when the charging polarity of the toner is minus (−), toner collecting capability can be increased by making the polarity of the bias applied to the fur brush opposite to the polarity of the toner, namely, into the plus (+) polarity, and increasing the intensity of the bias, but if the intensity of the bias is too much increased, the toner charged to the minus (−) polarity during the collection thereof is charged to the plus (+) polarity by charge injection or discharge, and this leads to the problem that the toner cannot by collected by the fur brush but is discharged onto the intermediate transfer belt. Therefore, the lower limit value of the bias setting of the electrostatic fur brush cleaning means is set to bias intensity by which the untransferred remaining toner can be collected, and the upper limit value thereof is set to bias intensity at which it is difficult for the toner to be reversed during the collection. As the result, the upper limit of the bias intensity puts an upper limit to the collecting capability.
Consequently, if an attempt is made to collect the remaining toner on the intermediate transfer belt by the above-described bias set valve and the method described in Japanese Patent Application Laid-open No. 2002-207403, when the toner is charged to the minus (−) polarity, a bias of the minus (−) polarity is applied to an upstream fur brush and therefore, almost all of the toner repulses and slips through, and a bias of the plus (+) polarity is applied to a downstream fur brush and therefore, the toner is collected, but due to the great amount of toner, much toner cannot be completely collected but slips through. Consequently, to collect all of the toner, it becomes necessary to make the remaining toner image on the intermediate transfer belt pass through the fur brush cleaning many times, and much time is required for the return.